Immune complexes (IC) are formed in the circulation or in tissue fluids as a consequence of the interaction between antigens and their corresponding antibodies. Many properties of IC, including clearance from the circulation, complement fixation, and adherence to phagocytes, depend on IC size and composition. The chronic presence of IC has been implicated in the pathology of several diseases including, systemic lupus erythematosus, rheumatoid arthritis, and neoplastic disease. The demonstration that activity which blocks the immune response to tumors can be removed by incubation of serum with protein A has motivated studies of plasma adsorption onto immobilized protein A in a number of tumor systems, with excellent success reported in treatment of lymphosarcoma and leukemia in FeLV-infected cats. Our research objectives are to: (1) determine the parameters that govern size, composition, and structure of IC, (2) Investigate the dependence of complex size and composition on binding and activation of complement, (3) understand the interactions between IC and the cells which bind them, and (4) define the reactions that occur upon contact of IC with immunoadsorbents such as immobilized protein A and conglutinin. Model IC constructed from two or more monoclonal antibodies and bovine serum albumin will be characterized in terms of molecular weight and size distribution, composition, overall structure, and mean hydrodynamic radius using electron microscopy, quasi-elastic light scattering, high performance size exclusion chromatography, radioimmunoassays, and mathematical models. Purified complement components will be incubated with model IC; the dependence of complement fixation and activation on complex size, structure, and composition will be measured. Similarly, IC will be incubated with macrophages and erythrocytes to measure binding and uptake, and with immunoadsorbents to analyze adsorption effects. These fundamental studies will provide the basis for experiments using IC isolated from sera of FeLV-infected cats. The proposed research will enhance our understanding of IC formation and behavior in the presence of complement and specific cells, and will provide a rational basis for future development and design of immunoadsorption techniques which may be applied diagnostically or therapeutically to a variety of disease states.